1. Field of the Invention
The present invention relates generally to video disc players and, more particularly, is directed to a video disc player and a method for reproducing a video disc in which a video signal is reproduced in a so-called scan reproduction mode.
2. Description of the Prior Art
Initially, let us explain an example of a prior art optical video disc player with reference to FIG. 1.
Referring to FIG. 1, there is provided a microcomputer 70 which controls the whole operation of this optical video disc player. There is shown an optical type video disc 10 on which there is recorded a signal, FM-modulated by a color composite video signal in the constant linear velocity (CLV) format. The optical video disc 10 is rotated by a spindle motor 51 and is servo-controlled by a spindle servo circuit 50 so that it is rotated in a constant linear velocity fashion.
There is provided a reproducing circuit 20 which reproduces a video signal. A photo pickup head (optical head) 21 for the disc 10 includes a laser light emitting element, a light receiving element for receiving a laser light emitted from the light emitting element, an objective lens, a tracking coil for moving an optical axis of the objective lens in the radial direction of the optical disc 10 and so on, though not shown. The photo pickup head 21 is moved in the radial direction of the optical disc 10 by a sled motor 44.
There is provided a tracking servo circuit 30, wherein one portion of an output signal from the photo pickup head 21 is supplied to a detecting circuit 31 which generates a tracking error voltage Vt. This tracking error voltage Vt is supplied through an amplifier 32 to the tracking coil of the photo pickup head 21, whereby the object lens is servo-controlled in tracking.
There is provided a sled servo circuit 40, in which the tracking error voltage Vt from the detecting circuit 31 is supplied to a low-pass filter 41, from which there is derived a DC component of the tracking error voltage Vt. This DC component is supplied through a switching circuit 42 and an amplifier 43 to the sled motor 44 to servo control the sled.
Accordingly, in the normal reproducing mode, the photo pickup head 21 is servo-controlled by the tracing servo circuit 30 and the sled servo circuit 40 so that the photo pickup head 21 correctly traces the tracks on the disc 10 to thereby produce a reproduced signal.
This reproduced signal is supplied through a playback amplifier 22 and a limiter 23 to an FM demodulating circuit 24, in which it is demodulated to provide a color composite video signal Sc. The color composite video signal Sc is supplied to a time base corrector 25, in which a jitter component is removed therefrom.
More specifically, the signal Sc from the demodulating circuit 24 is supplied to a charge coupled device (CCD) 251, and the signal Sc from the CCD 251 is supplied to a synchronizing separating circuit 252, from which there is derived a horizontal synchronizing pulse PBH2. The horizontal synchronizing pulse PBH2 and a synchronizing pulse REFH are supplied to a phase comparing circuit. The synchronizing pulse REFH has a reference horizontal frequency and is derived from a master reference signal generating circuit 61. The phase comparing circuit 253 derives a phase-compared output of the pulses PBH2 and REFH which it supplies to a low-pass filter 254 which generates a time base error voltage TBCE whose level changes in response to the phase difference between the pulses PBH2 and REFH. The time base error voltage TBCE is supplied to a voltage controlled oscillator (VCO) 255 as a control signal, and an oscillation signal from the VCO 255 is supplied to the CCD 251 as a clock signal.
Accordingly, when the TBC 25 is stabilized, the pulse PBH2 becomes a signal having a constant phase synchronized with the reference pulse REFH so that the video signal Sc from the CCD 251 at that time becomes a signal whose jitter component is removed. This video signal Sc is supplied through a switching circuit 28 to an output terminal 29.
At that time, in the synchronizing separating circuit 252, a vertical synchronizing signal PBV is separated from the video signal Sc and this pulse PBV is supplied to the servo circuit 50. Simultaneously, the generating circuit 61 derives a synchronizing pulse REFV of reference vertical frequency, and this synchronizing pulse REFV is supplied to the servo circuit 50, whereby the revolution of the motor 51 is controlled in such a manner that the pulse PBV is synchronized with the pulse REFV and a first spindle servo is performed.
Further, the video signal Sc from the demodulating circuit 24 is supplied to a synchronizing separating circuit 52, which derives a horizontal synchronizing pulse PBH1. This horizontal synchronizing pulse PBH1 is supplied to the servo circuit 50 along with the pulse REFH from the generating circuit 61 whereby the revolution of the motor 51 is controlled so that the pulse PBHI is synchronized with the pulse REFH and a second spindle servo is performed.
As described above, in the normal playback mode, the spindle servo is performed such that the reproduced synchronizing pulses PBV and PBHI are synchronized with the reference synchronizing pulses REFV and REFH. The operation of the video disc player in the normal playback mode is described so far.
In the above-described video disc player, when the photo pickup head 21 is moved in the radial direction of the optical video disc 10 at a speed higher than that of the normal playback mode by the sled servo circuit 40 while the objective lens within the photo pickup head 21 is servo-controlled in tracking by the tracking servo circuit 30, the objective lens tries to stay at the original position regardless of the movement of the photo pickup head 21.
When the photo pickup head 21 is moved to the control limit of the tracking servo, the objective lens performs a so-called track jump and again starts tracking the target track next to the jumped track. In practice, the tracking servo is turned OFF, and the track jump is forcibly performed.
Accordingly, even when the photo pickup head 21 is moved at a speed higher than that of the normal playback mode, the correct video signal Sc can be obtained intermittently, whereby a reproduced picture in the fast-forward-. or fast-rewind mode can be obtained by utilizing the correct video signal reproduced. In the following description, the above-mentioned operation mode will be referred to as the "scan mode" or "scan playback" mode.
In the scan playback mode, under the control of the microcomputer 70, the switching circuit 42 is changed to the opposite state to that shown FIG. 1, namely, the switching circuit 42 is connected to a fixed contact SCN, and also the signal generating circuit 61 generates a sled pulse SLDP at, for example, every 4 field periods. This sled pulse SLDP is supplied through the switching circuit 42 and the amplifier 43 to the motor 44, causing the photo pickup head 21 to be moved toward the inner peripheral or outer peripheral direction of optical video disc 10 at a speed higher than that of the normal playback mode. Further, a track jump pulse TJMP from the microcomputer 70 is supplied to the detecting circuit 31, allowing the objective lens within the photo pickup head 21 to perform the track jump.
Accordingly, till the next track jump after a track jump, the correct video signal Sc is obtained so that, similarly to the normal playback mode, the correct video signal Sc is obtained at the output terminal 29.
During the track jump, however, the correct video signal Sc is not obtained and only a noise signal is obtained so that the switching circuit 28 is connected to a fixed contact DUM by the microcomputer 70. Simultaneously, horizontal and vertical reference synchronizing pulses REFH and REFV, respectively, from the signal generating circuit 61 are supplied to a dummy signal generating circuit 62 which generates a pseudo-video signal Sq which is reproduced as, for example, a gray picture. The signal Sq is supplied through the switch circuit 28 to the output terminal 29.
Accordingly, in the scan playback mode, a reproduced picture based on the correct video signal Sc and the gray picture based on the pseudo-video signal Sq are alternately displayed, whereby the user can temporarily check a picture between the jumps in the fast-forward or fast-rewind mode.
When the scan playback is carried out as described above, the TBC 25 mal-functions. This will be described hereinafter.
As shown in the period just before time point t1 of FIGS. 2A to 2C, in the normal playback mode, the reference synchronizing pulse REFH and the reproduced synchronizing pulses PBHI and PBH2 are substantially the same in phase. Therefore, as shown in the period just before the time point t1 of FIG. 2E, the time base error voltage TBCE slightly fluctuates up and down around a central value Ec in response to the jitter component.
If the track jump is performed from the time point t1 to a time point t2 in order to perform the scan playback, the pulses PBH1 and PBH2 become noise components during this period. In practice, the jitter component is compensated for by the CCD 251 so that the pulse PBH2 is delayed from the pulse PBHI by a delay time of substantially one horizontal period (1H), which fact can be neglected in this description. Further, as shown in FIG. 2F, the low-pass filter 254, for example, is controlled by a control signal HOLD from the microcomputer 70, whereby the error voltage TBCE is held at a central value Ec from the time point t1.
When the track jump is ended at the time point t2 and the tracking is then stabilized, the pulses PBH1 and PBH2 are obtained at the next time point t3. However, the time point t3 at which the pulse PBH1 is obtained is random with respect to time point at which the pulse REFH is obtained so that, as shown in FIGS. 2A and 2B, they are not generally coincident with each other. If the time points of the two pulses PBHI and REFH are not coincident with each other, then the TBC 25 must compensate for the jitter component while incessantly absorbing the phase difference between the two pulses PBHI and REFH. Therefore, the TBC 25 needs a wide compensation range.
Consequently, in this example of the prior art, when the pulse PBH1 is obtained at the time point t3, the microcomputer 70 supplies a control signal HRES to the generating circuit 61 to reset the pulse REFH at the time point t3 as shown in FIG. 2D and make the pulse REFH the same in phase as that of the pulse PBH1. Accordingly, the pulse REFH is generated at each horizontal period as described hereinbefore. Further, the holding state of the TBC 25 is released from the time point t3 by the hold signal HOLD.
Therefore, it is to be appreciated that the above-mentioned correct video signal Sc is obtained from the time point t3 to the next track jump. In the scan reproduction of the CLV optical video disc, the relative velocities of the pickup head to the disc track at the track jump point and at the track after the track jump are different. In practice, the time point t3 is just behind the track jump so that the response of the spindle servo circuit 50 cannot follow the track jump. As a result, if the time point is just after a track jump, for example, in the fast-rewind mode direction, then the revolution speed of the optical video disc 10 is slower than the revolution speed needed by the track next to the jumped track and the revolution speed of the optical video disc 10 reaches the necessary revolution speed as the time passes.
Similarly, when a track jump in the fast-forward direction is performed, as shown in FIG. 2B, the cycle of the synchronizing pulse PBHI is longer than the reference value just immediately after the time point t3 and reaches a reference value as the time passes.
The TBC 25 compensates for the video signal Sc having the pulse PBHI such that the cycle of the pulse PBH2 equals the reference value from the time point t3 so that the phase relationship among the pulses PBHI, PBH2 and REFH exceeds the compensation range of the TBC 25. At the time, the error voltage TBCE is fixed to a maximum value Eu or minimum value Ed as shown in the period succeeding to a time point t4 of FIG. 2E with the result that the TBC 25 cannot carry out the correct operation. When the TBC 25 mal-functions as described above, a correct reproduced picture cannot be displayed.
When the scan reproduction is carried out as described above, if the optical video disc 10 is recorded according to the CLV format, the angular position at which the vertical synchronizing pulse PBV is recorded is slightly displaced from track to track as shown in FIG. 3 and a scan reproduced picture in which the synchronization is disturbed is obtained.
The above-mentioned phenomenon is described in our copending U.S. patent application Ser. No. 472,748, filed Jan. 31, 1990 and entitled "Video Disk Player," and will be summarized as follows.
Assuming that P is the track pitch (=1.67.times.10.sup.-6 (m)), R is the radius of the innermost periphery of the track (=55.times.10.sup.-3 (m)) and N is the track number (=1 to 54000), then the length L of N'th track is determined as: EQU L=2.pi.(R+P(N-1)) (m) (i).
Video signals of two fields are recorded in the innermost peripheral track of the video disc according to the CLV format and one track length is fixed as .pi.R so that the number F of the fields involved in the N'th track is expressed by the following equation (ii) ##EQU1## Since k=2P/R.div.60.73 (p.p.m.), N=1 yields F=2 fields, and N=54000 yields F.apprxeq.5.28 fields.
The value k in the equation (i) represents the amount of how much the angular position at which the vertical synchronizing signal is recorded is changed when one track jump is carried out. That is, the value k indicates the amount in which the phase of the reproduced vertical synchronizing signal is changed.
A relationship between the track and the vertical synchronizing pulse Vsync in FIG. 3 will be described more fully with reference to FIGS. 4A and 4C.
FIG. 4A shows the condition near a track portion where two video fields are recorded in one track, wherein vertical synchronizing pulses Vsync are aligned substantially in the radial direction of the disc. FIG. 4B shows the condition near a track portion in which three video signals are recorded in one track, wherein although the vertical synchronizing pulses Vsync are aligned substantially in the radial direction of the disc similarly to FIG. 4A, the phase of the reproduced vertical synchronizing pulses Vsync deviates by several 10s of percents from the phase of a reference vertical synchronizing pulse of the apparatus. This is because the spindle servo is applied so that the phase of the vertical synchronizing pulse in the recorded signal deviates considerably from the phase of the reference vertical synchronizing pulse. Therefore, in the scan mode, the photo pickup head can not be pulled in a servo controllable range of.+-.3% by the servo control. If the photo pickup head is jumped into the track of this area, then a picture cannot be immediately reproduced and so, this area is referred to as a dead zone track area.
FIG. 4C illustrates the condition near a track portion where 2.7 video fields are recorded in one track. As shown in FIG. 4C, the recorded positions of the vertical synchronizing pulses fluctuate around the recorded positions of the reference vertical synchronizing pulse. If the track shifting is carried out for at least 10 or more tracks near this track position, then at least the vertical synchronizing pulse of a certain track fails within a pull-in range of the servo control.
It is to be understood from FIGS. 4A to 4C that, when a track jump is carried out near the tracks of F=2, 3, 4 and 5, if the number of tracks jumped is small (9 tracks in FIGS. 4A and 4C), the phase change of the vertical synchronizing pulse in the reproduced video signal after the track jump is small as compared with that occurring before the track jump. It is needless to say that, even when the track jump is carried out near the tracks of F=2, 3, 4, and 5, if the number of tracks jumped is selected to be very large, then the phase change of the vertical synchronizing pulse after the track jump is carried out can be made large. In this connection, the number of tracks in which 2, 3, 4 and 5 fields of the video signal are recorded in one track is expressed by the following equation ##EQU2##
If F=2, 3, 4 and 5 is substituted into the above equation (iii), this yields N=1, N=16468, N=32935 and N=49402, respectively.
When a track jump is carried out near the track of F=2.5, 3.5, 4.5, . . . , even if the number of tracks jumped is small, the phase of the reproduced vertical synchronizing pulse after the track jump is considerably changed as compared with the change before the track jump. In the case of FIG. 4C, the phase of the vertical synchronizing pulse is considerably changed only by a jump of one track from N=11500, whereby the vertical synchronizing pulse enters a region of.+-.3%.
In the following description, the area in which the phase change of the vertical synchronizing pulse after the track jump is small and in which the phase of the reproduced vertical synchronizing pulse after a track jump of 10 or more tracks cannot match the phase of the reference vertical synchronizing pulse is referred to as a "dead zone track area."
Accordingly, if the dead zone track area of the video disc recorded according to the CLV format is reproduced in the conventional scan playback mode, a picture is reproduced wherein the vertical synchronization thereof is disturbed.
To avoid this defect, it is proposed that, in the scan playback mode, if the phase of the vertical synchronizing pulse contained in the reproduced video signal is continuously large (more than.+-.3%) relative to the phase of the reference vertical synchronizing pulse, the above synchronizing pulse is removed and a new vertical synchronizing pulse is inserted into a position in which a time series is continued. In this proposal, however, although the vertical synchronization is not disturbed, a black band corresponding to the removed vertical synchronizing pulse appears in the reproduced picture or the upper half portion and the lower half portion of the reproduced picture are reproduced in the picture screen in a vertically reversed fashion. These conditions are represented in FIG. 5 and FIGS. 6A and 6C, respectively. FIG. 5 illustrates the vertical synchronizing pulse PBV and the reference vertical synchronizing pulse REFV of the reproduced picture, whereas FIGS. 6A and 6C illustrate the examples of the reproduced pictures monitored on a television receiver.